Ink-jet ink prints have a lot of advantages over laser-jet prints because of easy to print with vibrant color prints that can be made low cost. The major problem with ink-jet printing is that its durability, e.g., water fastness, smear fastness and light fastness, is poor compared to laser prints. Water-soluble dyes or water-dispersible pigments are used in ink-jet inks for nozzle reliability and kogation reasons. In laser prints, no solvent or vehicle is used; only polymer-coated solid colorant particles are used to fuse with the media. Ink-jet inks use water-soluble or dispersible colorants. The property of the colorant remains the same, namely, water-soluble or dispersible even after printing. In addition to these problems, there are other disparities in performance properties. These properties include sharpness of the image, highlighter smear fastness and wet smudge fastness etc. Currently, no polymer additive is known that can be added to the ink to solve all the above issues. The present teachings herein address and overcome the above deficiencies and bring similar performance to ink-jet prints as laser prints using specific core-shell polymer additives.
Attempts have been made previously to make ink-jet ink prints having laser print type quality and durability. None of the prior art deficiencies have been solved simultaneously with a single ink. The present teachings herein address the achieving of laser type durability, sharpness of the print, water-fastness, wet smudge-fastness, smear fastness and the like by the addition of a polymer additive to ink-jet ink. Previous attempts have been made to solve these issues. But previous inks containing polymers are not very shear stable and there is difficulty in keeping the nozzles functional over a long period of time or for a large number of prints. Recently, another patent application has been filed with USPTO in the name of the present inventor (U.S. patent application Ser. No. 09/956,431, filed Aug. 20, 2001), which demonstrates the improvement of shear stability of polymers. This type of polymer improves shear stability but the resulting smear fastness is not like that of laser type prints. There is therefore room to improve inkjet inks with a different kind of polymer additive to achieve laser type quality.
The synthesis of core-shell polymers is well known in polymer chemistry. Core-shell polymer synthesis and structures are described by K. Ishizu in Progress in Polymer Science, Vol. 23, pp. 1383-1408 (1998). The latex polymers that can be used in ink-jet pens are very few. A number of parameters have to be met by the latex polymer to be used in the ink. Among the most important parameters are that the polymers have to be compatible with the colorants and the co-solvents used in the ink without unduly changing the viscoelastic properties of the ink. The particle size of the polymer particles should not change with time in the presence of those co-solvents. The latex polymers should not form films in the nozzles and should remain indefinitely stably suspended in the ink. At the same time, the polymer particle should form a protective film after landing in the media. The latex particles described herein coalesce irreversibly, leading to the formation of film when the water concentration is reduced. Water can be absorbed by the media or diffused or evaporated with time to reduce its concentration. Alternately, one can apply heat to remove water faster. The latex polymers have to be shear-stable and be ejectable as fine droplets down to a volume of 0.2 pl. After printing on the print media, the latex polymers have to form a film that will protect the colorant to improve durability properties such as water fastness, smear fastness, dry rub resistance, and should also provide gloss to the print. Many patents disclose latex polymers that possess one or another of the foregoing properties, but none of the polymers known in the prior art meet all of the foregoing requirements.
Core-shell polymers are described in several patents and in the technical literature. However, many of them use one or more acid monomers in the core stage.
The prior art fails to teach the synthesis of hydrophobic core polymers with hydrophobic and hydrophilic shell materials, with particles greater than 50 nm with a cross-linked shell polymer. Further, the prior art does not teach polymers having any film-forming property of both the core and the shell polymers with shear stability. Thus, there is a need for core-shell polymers having these properties for use in ink-jet inks.